Hi All
It IS a big problem with the call reject via the power button.
I wrote HTC support regarding this and got the following answer:
Hi Jan, Thank you for contacting us regarding Desire. Unfortunately there is no option to disable the reject call feature using the Power button. We have not had any feedback regarding this yet, but if this becomes a general problem - as in customers complaining over this feature - we will pass it on to our technical department in order for them to possibly make a change / fix for the devise. I will pass oyur feedback on to my superior, and if we have a You are welcome to contact us again for further assistance.
So I guess that I can only recommend that you all write to HTC Support regarding the Power Button issue - both the reject and the immidiate power on when the device is powered off.